msm: kgsl: Change semantics of the GMU rmw function

To make code easier to understand, change the semantics
of _gmu_regrmw to match kgsl_regrmw and move it to the
same place.

CRs-Fixed: 2017390
Change-Id: If6b9b6f614d95e0e3e4f27d4b84d4479bb8a5a22
Signed-off-by: Kyle Piefer <kpiefer@codeaurora.org>